Goku is a character who has changed dramatically since his early Dragon Ball days. Dragon Ball started as a humble adventure-gag manga, loosely adapting Journey to the West, but as time went on, the manga changed gears, becoming a far more ambitious sci-fi fantasy epic.
Along the way, Goku's character has changed so drastically that it might be hard to even recognize him as the monkey-tailed hero who set out on his adventure all those years ago. The following five events are the most important moments in terms of Goku's development as a character.
1. Goku Meeting Bulma
Goku's first union with Bulma is what sparks Goku's trip into a larger world. Bulma asking Goku to help her find the Dragon Balls brings Goku out of the home of Grandpa Gohan, pushing him off to meet the likes of Master Roshi, Yamcha, Oolong and Emperor Pilaf. Without meeting Bulma, Dragon Ball simply would never have happened.
This forces Goku, who until this point was very secluded, to learn about the outside world. Goku comes into the world incredibly naive, both in terms of social skills and life knowledge. He is shocked, for example, to learn Bulma doesn't have the same parts he has. Meeting Bulma was vital to giving Goku any understanding of the world around him.
2. Goku Losing Krillin (The First Time)
Many people remember when Goku lost Krillin in the fight against Frieza, but arguably the first time Krillin died was more impactful to Goku's growth and the trajectory of the Dragon Ball franchise as a whole. Krillin was one of the many fighters slain by the Demon King Piccolo's henchman, Tambourine. While this event in the immediate story motivates Goku to fight Piccolo, it also sparks a sense of darkness in Dragon Ball that would carry on into Dragon Ball Z and beyond.
This is one of the first times Goku develops a complicated vengeful relationship with a villain. Goku didn't really hold much hatred toward Emperor Pilaf or even the Red Ribbon Army in earlier sagas, but Goku shows real anger and desperation during the fight with Piccolo. This is made worse when the Demon King starts taking down Goku's friends, one by one. This is where the Goku we see most prominently throughout Dragon Ball Z first emerges.
3. Goku Marries Chi-Chi
When most fans think back to the 23rd Tenkaichi Budokai -- the final arc in the original Dragon Ball -- most remember the battle between Goku and Piccolo. It is often cited as a turning point in Dragon Ball's history. However, the real turning point happened earlier in the tournament when Goku agreed to fulfill his ignorant childhood promise to marry Chi-Chi.
With Chi-Chi, Goku would go on to have two sons, continuing his family line. Goku becoming a father is a massive turning point in his arc. While Goku is still a combat obsessed warrior, he finds himself driven to protect his son. Goku is mostly indifferent to Raditz until he steals Gohan from him. Years later, Goku, the man obsessed with proving himself stronger than his opponents, surrenders to Cell because he knows his son, Gohan, can take the villain down on his own.
Marrying Chi-Chi introduces several new variables in Goku's life, forcing him to juggle his obsession with fighting and his love for his family. Goku's balance between family life and fighting really puts new levels of responsibilities on Goku's shoulders that he deals with for the rest of his life -- usually, for the better.
4. Goku Dying at Piccolo's Hands, Fighting Raditz
Many might point to the introduction of Raditz as the point where Goku's arc changes. However, while Raditz's arrival serves as a great turning point for the story of Dragon Ball -- being where the anime split off into Dragon Ball Z -- Goku's character doesn't change drastically until he sacrifices himself to ensure Raditz is slain by Piccolo's Special Beam Cannon.
Dying transforms Goku's entire outlook on life. It introduces Goku to a far larger world, giving him a far larger perspective on the universe. It also pushes Goku to near-cosmic levels of power thanks to his training under King Kai. It is thanks to King Kai that Goku's focus shifted from martial arts prowess to intense power-up and beam-struggles. This is where Goku learns the Kaio-Ken and Spirit Bomb techniques. Goku had already become the strongest fighter on the planet. King Kai showed him new plateaus he could reach.
5. Goku Turning Super Saiyan
One of the biggest turning points in Goku's entire arc is his transformation into a Super Saiyan while fighting Frieza on Namek. Leading up to this, Goku is already reaching the climax of all his character development up until this point. He has the friends he met along his decade-spanning adventure on his side, he has the cosmic powers he learned from King Kai and he's facing off against the strongest entity in the universe.
However, when Frieza kills Krillin, Goku is pushed into a pure rage that makes him transform. The Super Saiyan transformation might have been cheapened somewhat by numerous follow-up transformations, but the original transformation is where Goku's entire perspective of power shifts, with his focus becoming reaching new forms in order to ascend to new power levels. At this point, we see another culmination for Goku: reaching the peak potential of his species -- or at least, the peak until the reveals of Super Saiyan 2, Super Saiyan 3, Super Saiyan 4, Super Saiyan God and so on...
In the episodes of Dragon Ball Z that followed Goku's initial transformation, we see a Goku now focused on perfecting this new form, learning to master and harness mythic transformations. In turn, he inspired those around him to pursue his levels of power. By transforming, Goku became a beacon for all those around him to reach. This is the point where Goku went from a strong fighter to the greatest warrior in the cosmos.